The privileges of a UK CPL and ATPL prohibit flight under IFR in Class A, B and C airspace unless you hold a valid IR. (ANO Sched 8)Therefore you are not prohibited from flying IFR in any other Class of Airspace. This inbuilt privilege is not a rating and therefore needs no revalidation.
The reference to having the privileges of a PPL with IMC rating is largely irrelevant as the very same privilege is built into the licence. Art 25 states that IMC Rating privileges are not valid unles you have a valid Certificate of Test!
So to sum up, the privilege to fly IFR in Classes D,E, F and G airspace is a basic licence privilege for the UK CPL and ATPL holder and the reference to the IMC rating is a total red herring.